Britons have been warned that the coming months will be 'challenging' as coronavirus cases show no sign of slowing down.
Downing Street insisted it did not have any immediate plans to bring back restrictions but admitted it would keep a 'close watch' on the situation.
The UK is currently recording nearly 43,000 new Covid cases every day on average with levels of infection now almost on par with the worst of the second peak in January.
Hospitalisations and deaths remain much lower but there are concerns that even a small rise could push the NHS to the brink this winter when it tries to juggle high levels of flu and usual seasonal pressures.
There are also fears that uptake of Covid booster jabs may be too slow to prevent a hospital surge this winter, with only half of eligible over-80s having receiving a third dose since the programme launched a month ago.
Professor Andrew Hayward, an epidemiologist and key Government adviser, said the situation was 'concerning' and there was 'huge potential for the NHS to come under a lot of pressure'.
The Prime Minister's official spokesman said No10 was not considering rolling back compulsory face masks or introducing vaccine passports as part of the Covid winter 'Plan B' just yet.
They added: 'We obviously keep very close watch on the latest statistics. We always knew the coming months would be challenging.
'What we are seeing is case rates, hospitalisations and deaths still broadly in line with the modelling as set out a few months back now.
Yesterday the UK reported 45,140 cases of coronavirus, bringing the total number of infections to 8,449.165
The number of Covid deaths in the UK yesterday was 57, down 61 per cent on last Sunday
'The vaccination programme will continue to be our first line of defence, along with new treatments, testing and public health advice. But we will obviously keep a close watch on cases.
'But it is thanks to our vaccination programme that we are able to substantially break the link between cases, hospitalisations and deaths.'
The spokesman said the success of the vaccines meant 'we are able to be one of the most open economies in Europe, which is benefiting the public and indeed businesses as well'.
Natural immunity IS just as good as being jabbed, official figures show
Recovering from Covid offers just as good protection as getting two doses of any vaccine, official figures suggest.
An Office for National Statistics' (ONS) report published today found unvaccinated Britons who catch the Delta variant are around 71 per cent less likely to test positive for a second time.
It estimated the risk of infection is slashed by approximately 67 per cent in people given two doses of Pfizer or AstraZeneca's jabs.
The ONS said there was 'no evidence' vaccines offered more immunity than catching Covid itself, despite a number of other studies showing the opposite.
The findings are based on more than 8,000 positive tests across Britain between May and August, when the Delta variant became dominant.
Scientists are still trying to untangle exactly how long naturally-acquired and vaccine immunity lasts.
Protection from the jabs appears to dip at around five months, which is why Britons over the age of 50 are being offered booster doses this autumn.
But the duration of natural immunity remains somewhat of a mystery, made more complicated by the rise of new variants.
Prof Hayward, a member of the Sage scientific advisory panel, told BBC Radio 4's World At One: 'I think it's concerning that we've got very high rates of infection and higher rates of hospitalisation and mortality than many of our European counterparts.'
He said waning immunity is 'probably part of' the reason infections are currently high, adding there is 'some evidence' protection against infection is beginning to wear off and 'probably some evidence' protection against severe disease is waning to a lesser extent.
Professor Hayward added: 'We shouldn't be complacent because there is still huge potential for the NHS to come under a lot of pressure and for there to be a lot of unnecessary deaths.
'So we need to get the vaccination rates up and we need to be prepared potentially to think about other measures if things do get out of control.'
Downing Street said 'different countries are potentially at different stages of their vaccination programmes and have different measures in place so it's difficult to compare and contrast'.
The Prime Minister's spokesman added: 'What's important is we strike the right balance between protecting lives and livelihoods.'
ONS figures suggest that around one in 10 schoolchildren in Years Seven to 11 in England was estimated to have Covid in the previous week, the highest positivity rate for any age group.
But analysis by the PA news agency suggested there was low take-up of Covid-19 jabs among 12-15 year-olds.
In some areas the rate of vaccine uptake is as low as 5%, while only 15 local authorities in England have managed to give a first jab to at least a quarter of 12 to 15-year-olds, data shows.
The picture is very different in Scotland, where young people can also receive doses of the jab in drop-in vaccination centres, as the take-up is already over 50% in half of local authority areas.
There have been calls for vaccines to be offered to under-16s in walk-in centres rather than in school in order to boost take-up.
James Bowen, director of policy for school leaders' union NAHT, said: 'Allowing 12-15 year olds to attend walk-in vaccination centres would be a sensible decision.'
The Prime Minister's spokesman said: 'In the first instance we are allowing for the jabs to take place through the school immunisation services, this is the long-standing approach that has been used for flu and HPV jabs.
'We are working very closely with schools, we are going to keep the programme for 12 to 15-year-olds under review.'
The spokesman said a number of factors could be behind the problems with getting jabs into children's arms, potentially including 'abhorrent' abuse and misinformation from protesters at school gates.
'It is completely unacceptable for anyone to direct abuse or misinformation towards parents, teachers or indeed children,' the spokesman said.
Meanwhile, French pharmaceutical firm Valneva, whose UK contract for vaccines was cancelled last month, has reported positive results from its Covid-19 trial.